Note sheet metal panel anchors under format 15 (#482)

Claude-Session: https://claude.ai/code/session_01HWLPLy7ug7URSK5WybjRNa

Co-authored-by: Claude Opus 5.5 <[email protected]>

crates/doc/src/format.rs

@@ -47,7 +47,8 @@
 //!   Pieces a feature splits a body into are named after their faces; features saved before
 //!   keep the kernel's shell order (`shell_piece_order`), which their stored body names mean.
 //! - 15: sketch face anchors and face bindings can name a cylinder primitive's cap
-//!   (`cylinder_cap`), and sketches can link lines to their own axes (`sketch_axis`). Version 14
+//!   (`cylinder_cap`), sketches can link lines to their own axes (`sketch_axis`), and sketch
+//!   anchors can name a sheet metal panel side (`sheet:<body>:<panel>:<side>`). Version 14
 //!   designs need no transformation; the version stamp lets older readers request an update
 //!   instead of encountering an unknown certificate or link kind.